Nerve cells

The biology of nerve cells:

• The neuron doctrine - the neuron is the fundamental building block and elementary signaling unit of the brain.
• The ionic-hypothesis - focuses on the transmission of information within the nerve cell. It describes the mechanisms whereby individual nerve cells generate electrical signals, called action potentials, that can propagate over a considerable distance within a given nerve cell.

The chemical theory of synaptic transmission - focuses on the transmission of information between nerve cells. It describes how one nerve cell communicates with another by releasing a chemical signal called a neurotransmitter. The second cell recognizes the signal and responds by means of a specific molecule in its surface membrane called a receptor.

Despite their complex shape, nerve cells are single, coherent entities. The fine processes surrounding them are not independent but emanate directly from the cell body. The entire nerve cell, including the processes, is fully enclosed by a surface membrane, consistent with the cell theory.

Cajal went on to distinguish the following parts of the neuron:

• a nucleus
• a single axon - emerges at one end of the cell body and can extend up to several feet. Often splits into one or more branches along its length, with many tiny axon terminals at the end of each branch. Send information to other cells
• dendrites - emerge on the opposite side of the cell body. They branch extensively (up to 40 branches in the human brain), forming a treelike structure that grows out from the cell body and spreads over a large area. Receives signals from other nerve cells.
• synapses - sites of connection between an axon and a dendrite. Communication across them has three components
  • The presynaptic terminal of the axon that sends a signal
  • The synaptic cleft or gap
  • The post-synaptic site on the dendrite that receives the signal

Typically, a single neuron makes contact through its many presynaptic terminals with the dendrites of many target cells and can disseminate the information it receives widely to different target neurons, sometimes located in different regions of the brain.

Conversely, the dendrites of a target nerve cell can receive information from the presynaptic terminals of a number of different neurons and thus integrate information from a number of different neurons, even those located in different areas of the brain

There are three major classes of neurons:

• Sensory neurons - which are located in the skin and in various sense organs, respond to a specific type of stimulus from the outside world and send this information to the brain.
• Motor neurons send their axons out of the brain stem and spinal cord to effector cells, such as muscle and gland cells, and control the activity of those cells
• Interneurons - the most numerous class of neurons in the brain, serve as relays between sensory and motor neurons